Effects of agricultural and tillage practices on isotopic signatures and fluxes of organic and inorganic carbon in headwater streams

Abstract

The amounts and characteristics of carbon (C) transported by streams and rivers are strongly connected to attributes of their associated watersheds. However, the factors controlling how different land uses influence the sources and inputs of organic and inorganic C to headwater streams are not fully understood. In order to assess how land use practices specifically influence headwater stream C, the concentrations and isotopic (natural δ13C and ∆14C) signatures of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate organic C (POC) were measured between October 2008 and August 2009 in streams of six small watersheds of differing land use. Bayesian mixing models were used to estimate contributions of potential C sources to stream DIC, DOC, and POC pools. Mixing model results indicate that sources of C to streams in tilled and non-tilled corn watersheds were dominated by C4 plant biomass and soil organic C. In all other watershed types stream C was dominated by C3 plant biomass. In addition, δ13C and Δ14C values of forested stream C were unique from values in the corn, pasture, and large mixed use watersheds, and showed greater contributions from modern-aged C3 biomass. Relative to other watershed types, tilled corn agriculture showed the greatest effect on both the sources and amounts of stream C. In the tilled corn watersheds, total C (DIC + DOC + POC) fluxes were 314% higher than in the non-tilled corn watershed and 39–76% higher than in all other watersheds. Thus, land use and agricultural practices can serve as strong controls over the sources and fluxes of organic and inorganic C to streams.